In recent years, in order to achieve both of fuel economy improvement and collision safety of an automobile, there is a growing need for a high strength steel sheet having the tensile strength of 590 MPa or more, 780 MPa or more, and particularly 980 MPa or more as a material for structural components, and the application range thereof is widening. However, because the variation in the mechanical property such as the yield strength, tensile strength, work hardening index, and the like of the high strength steel sheet is large compared to that of a mild steel, there are problems that the dimensional accuracy of the press formed product is hardly secured because the spring-back quantity changes in press forming, and that the life of the press forming tool is shortened because the average strength of the steel sheet should be set high in order to secure the required strength of the press formed product even when the strength varies.
In order to solve such problems, various trials have been made with respect to suppressing the variation in the mechanical property in the high strength steel sheet. The cause of generation of the variation in the mechanical property as described above in the high strength steel sheet can be attributed to the fluctuation in the chemical composition and the variation of the manufacturing condition, and following proposals have been made with respect to methods for reducing the variation in the mechanical property.
[Prior Art 1]
For example, in Patent Literature 1, a method for reducing the variation in the mechanical property is disclosed in which the steel sheet is made a dual-phase microstructure steel having ferrite and martensite in which A defined by A=Si+9×Al satisfies 6.0≦A≦20.0, in manufacturing the steel sheet, recrystallization annealing/tempering treatment is executed by holding at a temperature of Ac1 or above and Ac3 or below for 10 s or more, slow cooling at a cooling rate of 20° C./s or less for 500-750° C., rapid cooling thereafter at a cooling rate of 100° C./s or more to 100° C. or below, and tempering at 300-500° C., thereby A3 point of the steel is raised, and thereby the stability of the dual-phase microstructure when the rapid cooling start temperature that is the temperature of the slow cooling completion time point fluctuates is improved.
[Prior Art 2]
Also, in Patent Literature 2, a method is disclosed in which the variation in the strength is reduced by that the relation between the tensile strength and the sheet thickness, carbon content, phosphorus content, quenching start temperature, quenching stop temperature, and tempering temperature after quenching of the steel sheet is obtained beforehand, the quenching start temperature is calculated according to the target tensile strength considering the sheet thickness, carbon content, phosphorus content, quenching stop temperature, and tempering temperature after quenching of the steel sheet of the object, and quenching is executed with the quenching start temperature obtained.
[Prior Art 3]
Also, in Patent Literature 3, there is disclosed a method for improving the variation in the elongation property in the sheet width direction by soaking at over 800° C. and below Ac3 point for 30 s-5 min, thereafter executing the primary cooling to the temperature range of 450-550° C., then executing secondary cooling to 450-400° C. with a lower cooling rate than the primary cooling rate, and holding thereafter at 450-400° C. for 1 min or more in the annealing treatment after cold-rolling the hot-rolled steel sheet in manufacturing a steel sheet having the microstructure including 3% or more of the retained austenite.
[Prior Art 4]
Also, in Patent Literature 4, there is disclosed a method for improving the drawability of a high strength hot-dip galvanizing-coated steel sheet by achieving the microstructure including a ferrite phase having the average grain size of 10 μm or less and a martensitic phase having the volumetric fraction of 30-90% in which the ratio of the sheet thickness surface layer hardness with respect to the sheet thickness center hardness is 0.6-1, the maximum depth of the crack and the recess developing from the boundary face between the coating layer and the steel sheet to the inside on the steel sheet side is 0-20 μm, and the area ratio of the flat section other than the crack and the recess is 60%-100%.
The prior art 1 described above is characterized to suppress a change in the microstructure fraction caused by the fluctuation in the annealing temperature by increasing the addition amount of Al and raising Ac3 point, thereby expanding the dual-phase temperature range of Ac1-Ac3, and reducing the temperature dependability within the dual-phase temperature range. On the other hand, the invention of the present application is characterized to suppress the fluctuation in the mechanical property caused by the change in the heat treatment condition by equalizing the fraction and the hardness of the hard and soft phases of the steel sheet surface layer section and the inside. Accordingly, the prior art 1 described above does not suggest the technical thought of the invention of the present application. Also, because the prior art 1 described above requires to increase the addition amount of Al, there is also a problem of an increase in the manufacturing cost of the steel sheet.
Further, according to the prior art 2 described above, the quenching temperature is changed according to the change in the chemical composition, therefore the variation in the strength can be reduced, however the microstructure fraction fluctuates among the coils, and therefore the variation in elongation and stretch flange formability cannot be reduced.
Furthermore, although the prior art 3 described above suggests reduction of the variation in elongation, reduction of the variation in stretch flange formability is not suggested.
Further, according to the prior art 4 described above, with the aim of improving the press formability, the average grain size of the ferrite phase is specified to be 10 μm or less and the hardness ratio of the steel sheet surface layer and the center is specified to be 0.6-1. However, because the grain size of the ferrite phase is specified only by the average value, when there is a large variation in the magnitude of the size of each ferrite grain, improvement of the press formability cannot be expected. Further, although the hardness ratio of the steel sheet surface layer and the center is specified, a large/small relationship of the hardness and the deformability of the hard and soft phases do not agree to each other. For example, between a case where the fraction of the hard phase tempered inferior in deformability is high and a case where the fraction of the soft phase excellent in deformability is high, even when the hardness is the same, the press formability is different, and therefore it is supposed that the variation occurs in the degree of improvement of the press formability even though both cases are effective in improvement of the press formability.
Further, in general, in order to manufacture structural components for an automobile using a high strength steel sheet, complicated press forming and bending work are executed, however, because a similar work is executed also for the high strength steel sheet of 780 MPa or more, particularly 980 MPa or more, not only the ductility and stretch flange formability but also excellent bendability is required.
In the meantime, in bending the steel sheet, a large tensile stress is generated in the circumferential direction in the surface layer section on the bending outer periphery side and a large compressive stress is generated in the circumferential direction in the surface layer section on the bending inner periphery side. Therefore, it is known that, by arranging a soft layer in the surface layer section of the steel sheet, these stresses are relaxed and the bendability is improved. As such a high strength steel sheet provided with a soft layer in the surface layer section of the steel sheet, such proposals as described below have been made.
[Prior Art 5]
For example, in Patent Literature 5, an ultra-high strength cold-rolled steel sheet is disclosed which contains C: 0.03-0.2%, Si: 0.05-2% or less, Mn: 0.5-3.0%, P: 0.1% or less, S: 0.01% or less, SolAl: 0.01-0.1%, and N: 0.005% or less, with the remainder consisting of Fe and inevitable impurities, in which a soft phase with the volumetric ratio of ferrite by 90% or more and the thickness of 10-100 μm is provided in the steel sheet surface layer, and the microstructure in the center section has tempered martensite with the volumetric ratio by 30% or more with the remainder being the ferrite phase.
[Prior Art 6]
Also, in Patent Literature 6, a high strength automobile member is disclosed which is characterized that the thickness of the surface layer is 1 nm-300 μm, the surface layer is a decarburized layer mainly of ferrite, the chemical composition of the inner layer steel contains C: 0.1-0.8% and Mn: 0.5-3% in mass %, and the tensile strength is 980 N/mm2 or more.
The prior art 5 described above is to attempt to improve the bendability by that two step cooling is executed after annealing combining cooling of the steel sheet surface layer first by slow cooling and cooling of the entire steel sheet next by rapid cooling, thereby the microstructure is made different between the surface layer and the center section, and a soft layer generally composed of ferrite only is formed in the steel sheet surface layer. However, according to this technology, crystal grains are liable to grow during annealing, and in the surface layer particularly, ferrite grains whose size is non-uniform compared with the microstructure in the center section are liable to be formed. When the size of the ferrite grains becomes non-uniform, not only the bendability itself deteriorates but also conspicuous unevenness is formed on the surface of a strong working section, and therefore a problem of deterioration of the surface shape also occurs.
Further, the prior art 6 described above is to attempt to reduce the sensitivity with respect to the delayed fracture by that the thickness of the surface layer is made 1 nm-300 μm, the surface layer is made a decarburized layer with 50% or more of ferrite in terms of mass %, and thereby the dehydrogenizing rate after hot stamping is significantly increased. Here, the inner layer is rapid-cooled after hot stamping and is transformed into a microstructure mainly formed of martensite, therefore, even though deformation may be followed during hot stamping, in cold working, bending work is difficult because the property of the surface layer and the inner layer is extremely different from each other.